1. Field of the Invention
The present invention relates to a photoconductor using laser light or other coherent light as a writing light, and to an image forming apparatus and a cartridge for an image forming apparatus using the photoconductor.
2. Description of the Related Art
An electrophotographic process by use of coherent light, such as laser light, as a writing light, is widely used for the formation of digital images such as in copying machines, printers and facsimile apparatus.
In an electrophotographic process using coherent light as a writing light, an image including light and shade stripes (hereinafter referred to as interference fringes) is formed due to the interference of the coherent light within a photoconductive layer of the photoconductor. Such light and shade stripes are generated by the writing light being intensified when the photoconductor satisfies the relationship of 2nd=mλ wherein n is the refractive index of a charge transporting layer at the wavelength of the writing light, d is the thickness of the charge transporting layer, λ is the wavelength of the writing light, and m is an integer. More specifically, when λ is 780 nm and n is 2.0, one set of light and shade stripes (interference fringes) appears at each change of 0.195 μm in the thickness of the charge transporting layer. In order to remove interference fringes completely, it is necessary to reduce the deviation of the thickness of the charge transporting layer to less than 0.195 μm in the entire image formation area. However, it is economically extremely difficult to produce a photoconductor with such a small deviation of the thickness of the charge transporting layer, so that various alternative techniques have been proposed to control or reduce the formation of interference fringes in images.
For example, Japanese Patent Application Laid-Open (JP-A) No. 57-165845 proposes a photoconductor comprising a support made of aluminum, a charge transporting layer formed on the support, a charge generating layer comprising amorphous silicon (a-Si) formed on the charge transporting layer, and further comprising a light absorption layer on the aluminum support to remove the mirror reflection of the aluminum support, thereby preventing the formation of interference fringes in images. The light absorption layer on the aluminum support is extremely effective for preventing the formation of interference fringes in the image with the photoconductor using the charge generating layer comprising a-Si with the layer structure of the aluminum support/charge transporting layer/charge generating layer as mentioned above. However, for an organic photoconductor with a layer structure of aluminum support/charge generating layer/charge transporting layer in general use, the provision of the light absorption layer on the aluminum support is not so effective for preventing the formation of interference fringes in the image.
JP-A No. 07-295269 discloses a photoconductor with a layer structure of aluminum support/undercoat layer/charge generating layer/charge transporting layer, with the provision of a light absorption layer on the aluminum support for preventing the formation of interference fringes in the image. However, the photoconductor with this layer structure cannot completely prevent the formation of interference fringes in the image.
Japanese Patent Application Publication (JP-B) No. 07-27262 discloses an image forming apparatus comprising a photoconductor and an optical system. The photoconductor comprises a cylindrical support which has such a convex cross section that is formed by superimposing a sub-peak on a main peak, when the cylindrical support is cut by a plane which includes the axis of the cylindrical support. The optical system uses a coherent light beam with a beam diameter which is less than one period of the main peak for exposure. In some photoconductors, the formation of interference fringes in the image can be controlled to some extent by use of the above-mentioned support. However, many photoconductors cannot prevent the formation of interference fringes in the image even though the above-mentioned support is used.
JP-A No. 10-301311 discloses a photoconductor including a photoconductive layer supported on a support, in which the center-line surface roughness Ry of the support is one half or more of the wavelength of the writing light beam so as to prevent the formation of interference fringes with respect to a writing light with a wavelength of 650 nm or more. The photoconductor may often reduce interference fringes when used in an image forming apparatus having a low resolution or having a relatively large spot diameter of writing light beam. However, when the spot diameter of the writing light beam is reduced so as to improve the resolution, interference fringes are unavoidably formed. The surface roughness Ry can properly represent magnitude of average unevenness of a profile curve composed of only waves with similar amplitudes. However, an actual profile curve of a photoconductor is composed of a multiplicity of waves of greatly different wavelengths and amplitudes. Minute waves superimposed on waves with large amplitudes are cancelled in calculating Ry and thus are not reflected in Ry at all. Ry is thereby no appropriate as a parameter for representing minute unevenness or roughness.
When an image forming apparatus with high resolution is used, even if the surface roughness of the support is defined by conventionally employed parameters such as maximum height (Rmax), and ten-point average roughness (Rz), there cannot be determined the conditions under which the formation of interference fringes can be completely prevented.
Photoconductors in which surface roughness of an intermediate layer and/or an outermost layer is specified are known.
For example, JP-A No. 2001-265014 discloses a photoconductor in which a profile curve at the interface of the photoconductive layer on the side of the support is specified according to Fourier analysis to avoid interference fringes. Specifying the profile curve according to Fourier analysis is very appropriate, and the photoconductor can substantially completely suppress the formation of interference fringes. However, when the photoconductor is used in an image forming apparatus having a photoconductor and an electrostatic charger arranged at a distance from the photoconductor of 100 μm or less, it often invites images with voids due to, for example, discharge breakdown. Such an image forming apparatus having a photoconductor and an electrostatic charger arranged close to the photoconductor is configured so as to reduce the formation of ozone, NOx, and other oxidizing substances upon electrification and is therefore environmentally friendly used.
JP-A No. 06-138685 discloses a photoconductor including a conductive support having a ten-point surface roughness Rz of 0.01 to 0.5 μm and a surface protective layer having an Rz of 0.2 to 1.2 μm. However, a surface protective layer is generally poor in hole transferring ability so that the photoconductor tends to cause an increase in electric potential of a latent image and to produce an unclear image by influences of, for example, ion species generated by electrification, oxidizing or reducing gas, and/or humidity. It is extremely difficult to specify an Rz to eliminate interference fringes completely. When the image forming apparatus has a high image writing resolution, image defects such as interference fringes tend to occur.
JP-A No. 07-13379 discloses a photoconductor including an intermediate layer and a surface protective layer for the purpose of preventing interference fringes such as moire. To prevent white voids in a solid pattern, the intermediate layer and the surface protective layer have specific ten-point surface roughness Rz of 1.0 μm or less. However, the Rz for each layer is not disclosed to be effective to prevent interference fringes such as moire.
JP-A No. 08-248663 discloses a photoconductor including a support having a surface roughness of 0.01 to 2.0 μm, and an outermost layer having a surface roughness of 0.1 to 0.5 μm and containing inorganic particles having an average particle diameter of 0.05 to 0.5 μm. However, it is not specified what kind of surface roughness is the surface roughness of the support and the outermost layer. As is described above, conventional parameters of surface roughness include Rmax, Rz and Ra. It is well known that measured surface roughness values obtained from a profile curve at the surface of a solid largely vary depending upon the parameters adopted and upon the measurement conditions such as measurement length. When the surface roughness of the support, and the surface roughness of the surface protective layer are specified as Rz defined in, for example, Japanese Industrial Standards (JIS), interference fringes occur in many cases, and such specifying cannot completely prevent such interference fringes. Moreover, even with a photoconductor having the same surface roughness, the degree of interference fringes varies depending upon the image writing resolution of the image forming apparatus.
The interference fringes can be prevented in many cases by roughening the surface of a support and/or a photoconductor, although means for reliably inhibiting image defects such as interference fringes has not yet been found. Moreover, even with the same photoconductor, the degree of interference fringes varies depending upon the resolution of the image forming apparatus, and the wavelength of the writing light. With the known techniques, it is impossible to produce images free of interference fringes while retaining other desired image qualities. It is also necessary to design, with a try-error technique, a desired photoconductor suited for a specific image forming device.
An excessively roughened surface of a photoconductor and/or of a conductive support may often invite white voids due to discharge breakdown, as described above. Accordingly, a demand has been made on an image forming technique that can inhibit both the interference fringes and discharge breakdown.